The low-frequency flattening of the radio spectrum of giant H II regions in M 101

¹ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
² ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, Dwingeloo 7991 PD, The Netherlands
³ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), Universitätsstrasse 150, 44801 Bochum, Germany
⁴ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁵ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Obserwatorium Astronomiczne Uniwersytetu Jagiellońskiego, ul. Orla 171, 30-244 Kraków, Poland
⁸ Chalmers University of Technology, Onsala Space Observatory, SE-43992 Onsala, Sweden
⁹ School of Physical Sciences and Nanotechnology, Yachay Tech University, Hacienda San José S/N, 100119 Urcuquí, Ecuador
¹⁰ Anton Pannekoek Institute for Astronomy at the University of Amsterdam, University of Amsterdam, P.O. Box 94249 1090 GE Amsterdam, The Nederlands

^⋆ Corresponding author; lovorka.gajovic@uni-hamburg.de

Received: 14 November 2024
Accepted: 6 February 2025

Abstract

Context. In galaxies, the flattening of the spectrum at low radio frequencies below 300 MHz has been the subject of some debate. A turnover at low frequencies could be caused by multiple physical processes, which can yield new insights into the properties of the ionised gas in the interstellar medium.

Aims. We investigate the existence and nature of the low-frequency turnover in the H II regions of M 101.

Methods. We study the nearby galaxy M 101 using the LOw Frequency ARray (LOFAR) at frequencies of 54 and 144 MHz, Apertif at 1370 MHz, and published combined map from the Very Large Array (VLA) and Effelesberg telescope at 4850 MHz.

Results. The spectral index between 54 and 144 MHz is inverted at the centres of H II regions. We find a significant low-frequency flattening at the centres of five out of six H II regions that we selected for this study.

Conclusions. The low frequency flattening in H II regions of M 101 can be explained with two different free-free absorption models. The flattening is localised in a region smaller than 1.5 kpc and can only be detected with high resolution (better than 45″). The detection of low frequency flattening has important consequences for using radio continuum observations below 100 MHz to measure extinction-free star-formation rates.